Offshore drilling and production platforms used for recovering oil from subterranean formations disposed beneath ocean water include a number of structural support legs for supporting a plurality of work areas. Generally, below the work areas, a plurality of hollow concrete multi-cell structures may sit on the seabed floor. The hollow concrete multi-cell structures may be large, in some cases including over eighty cells, each cell reaching volumes of thousands of cubic meters.
Previously, the hollow concrete multi-cell structures may have been used to separate hydrocarbons from water, store hydrocarbons, or otherwise collect a hydrocarbon source. As a function of operation, hydrocarbons may become trapped in the hollow concrete multi-cell structures. The hydrocarbons trapped in the hollow multi-cell structures are often referred to in the industry as “attic oil.”
At one time abandoned storage vessels could remain partially filled with residual hydrocarbons. One such source of abandoned hydrocarbons occurs in storage vessels awaiting decommission. Prior to decommissioning, the remaining hydrocarbons in the storage vessels must be removed. Additionally, to prevent contamination of the ecology around the hollow multi-cell structures, the decommissioning of storage vessels must occur in an environmentally clean manner.
While methods for removing oil from subterranean reservoirs are known to those skilled in the art, the methods are directed to the removal of oil from subsurface formations, and do not consider the constraints of removing oil from storage vessels, for example, above ground or on the seabed floor. One such method is disclosed by U.S. Pat. No. 4,676,314 (“the '314 patent”), hereby incorporated by reference herein. The '314 patent describes injecting air into the top of a subterranean formation that is filled with both oil and water. As the air displaces the water, the water flows out of the formation, the oil settles towards the bottom of the formation, and a well is dug to extract the oil which has settled to the bottom of the formation.
Another method is disclosed in U.S. Pat. No. 4,679,627 (“the '627 patent”), hereby incorporated by reference herein. The '627 patent describes injecting gas into a subterranean reservoir, forcing the oil to the bottom of the formation, and then generating pressure waves to release additional oil retained by the reservoir. The oil is then removed by drilling a well into the formation, and extracting the displaced oil from the bottom of the formation.
While the '314 patent and the '627 patent describe methods of injecting gas into a subterranean formation to recover oil trapped therein, the methods both involve drilling a well into the formation, an option that is not available when removing oil from a storage vessel on the seabed floor. Further, the prior disclosures remove the oil through an export location drilled through the top of a formation. On oil platforms, rather than being located on the top of the storage vessels, the export location is generally located below the hydrocarbon layer, thereby preventing removal though the simple pumping described in prior disclosures.
Accordingly, there exists a need for a method to extract hydrocarbons from storage vessels in an efficient, environmentally clean, and profitable manner.